ARTHROPODS IN RELATION TO PLANT DISEASE Transmission of Xylella fastidiosa to Grapevines by Homalodisca coagulata (Hemiptera: Cicadellidae) RODRIGO P. P. ALMEIDA AND ALEXANDER H. PURCELL 1 Department of Environmental Science, Policy and Management, University of California, Berkeley, CA 94720 J. Econ. Entomol. 96(2): 264Ð271 (2003) ABSTRACT PierceÕs disease (PD) of grapevines is caused by a xylem-limited bacterium Xylella fastidiosa (Wells, Raju, Hung, Weisburg, Mandelco-Paul, and Brenner) that is transmitted to plants by xylem sap-feeding insects. The introduction of the sharpshooter leafhopper Homalodisca coagulata (Say) into California has initiated new PD epidemics in southern California. In laboratory experi- ments, the major characteristics of H. coagulataÕs transmission of X. fastidiosa to grapevines were the same as reported for other vectors: short or absent latent period; nymphs transmitted but lost infectivity after molting and regained infectivity after feeding on infected plants; and infectivity persisted in adults. Adult H. coagulata acquired and inoculated X. fastidiosa in 1 h of access time on a plant. Inoculation rates increased with access time, but acquisition efÞciency (20% per individual) did not increase signiÞcantly beyond 6-h access. Estimated inoculation efÞciency per individual per day was 19.6, 17.9, and 10.3% for experiments where plant access was 1, 2, and 4 d, respectively. Freshly molted adults and nymphs acquired and transmitted X. fastidiosa more efÞciently than did older, Þeld-collected insects. H. coagulata transmitted X. fastidiosa to 2-yr-old woody tissues of grapevines as efÞciently as to green shoots. H. coagulata transmitted X. fastidiosa 3.5 mo after acquisition, demonstrating persistence of infectivity in adults. About half (14/29) of the H. coagulata from which we failed to culture X. fastidiosa from homogenized heads (with a detection threshold of 265 CFU/head) transmitted the pathogen to grape, and 17 of 24 from which we cultured X. fastidiosa transmitted. KEY WORDS glassy-winged sharpshooter, PierceÕs disease, vector, xylem Xylella fastidiosa (Wells, Raju, Hung, Weisburg, Man- delco-Paul, and Brenner) is a gram-negative, xylem- limited bacterium that causes disease in many crops, including PierceÕs disease (PD) of grapevines (Vitis vinifera L.) (Davis et al. 1978). Xylem sap-feeding insects such as sharpshooter leafhoppers (Cicadelli- dae: Cicadellinae) and spittlebugs (Cercopidae) transmit X. fastidiosa (Houston et al. 1947, Severin 1949, 1950). The bacterium is widespread in the Amer- ican tropics and subtropics and also has been reported from Taiwan and southeastern Europe (Purcell 1997). X. fastidiosa has a very broad host range, colonizing members of at least 28 plant families (Hill and Purcell 1995b). There is no curative treatment for grapevines with PD. Current control strategies in coastal Califor- nia are based on preventing entrance of vectors into vineyards during the spring by spraying riparian veg- etation with insecticides, and management of adjacent vegetation to eliminate hosts of X. fastidiosa and sharp- shooters (Goodwin and Purcell 1990, Purcell and Feil 2001). The introduction of Homalodisca coagulata (Say) into southern California in early 1989 (Sorensen and Gill 1996) drastically increased the threat of PD and related diseases to the stateÕs agriculture (Blua et al. 1999). H. coagulata appears to have been responsible for spreading oleander leaf scorch (also caused by X. fastidiosa) in the Los Angeles Basin (Purcell et al. 1999). In southern California, grape growers in Te- mecula Valley detected PD on a limited number of grapevines in 1997 for the Þrst time in that region, and associated it with presence of H. coagulata in neigh- boring citrus groves (Blua et al. 1999). The incidence of PD increased exponentially so that within 2 yr all areas of Temecula Valley were affected (Perring et al. 2001, Purcell and Feil 2001). Because it readily feeds on almond trees, H. coagulata also threatens to in- crease the importance of almond leaf scorch disease, which also is caused by X. fastidiosa (Davis et al. 1980). Virtually all sharpshooters and spittlebugs are vec- tors of X. fastidiosa (Purcell 1989). Although many studies have identiÞed vector species, only a few have determined transmission characteristics. Detailed transmission studies with Graphocephala atropunctata (Say), the principal X. fastidiosa vector in coastal California vineyards, revealed that X. fastidiosa is transmitted without any latent period (delay after acquisition) (Purcell and Finlay 1979) and for an in- 1 E-mail: purcell@nature.berkeley.edu. 0022-0493/03/0264Ð0271$04.00/0  2003 Entomological Society of America Downloaded from https://academic.oup.com/jee/article/96/2/264/2217700 by guest on 13 June 2022